Investigation of digital audio manipulation methods
Brendan Trevorrow · University of Southern Queensland ePrints (University of Southern Queensland) · 2013
This project investigates the application of signal processing techniques to three unique situations: 1) the extraction and suppression of a melody in a digital recording, 2)changing the tempo (time stretching) of a polyphonic musical recording without changing its pitch and 3) the reconstruction of audio waveforms which have been truncated (clipped) due to sample values exceeding full scale detection. Each of the applications under investigation requires the implementation of signal processing algorithms. The melody extraction/suppression application is performed through the implementation of digital filters which target the harmonic components of a musical note. Four filters are examined: the finite impulse response filter, the two pole resonant filter, the notch filter, and the infinite impulse response bandstop filter. The constant pitch time stretching application investigates two techniques: the phase vocoder and overlap-add. Finally the truncated waveform reconstruction application investigates the effectiveness of using low pass filtering and interpolation. All algorithms are implemented using a high level programming language and the performance of these algorithms is measured using a combination of quantitative and qualitative assessment. The melody extraction/suppression algorithms are shown to be capable of extracting and suppressing a melody to a certain extent, their effectiveness being dependent on the type of instrument which is being filtered. The truncated waveform reconstruction techniques proposed show only limited success, capable of removing clipping in simple waveforms with only minor amounts of clipping. The three time stretching techniques all manage to successfully change the speed of a digital recording without changing its pitch, however each technique introduces their own distinct audible artifacts.